Current mobile communication systems such as the GSM system are typically based on a cellular structure. While moving, mobile stations (MS) often cross cell boundaries and at the same time change base stations, i.e. perform a handover. A handover requires complex preparations, including e.g. synchronisation to the frequency and transmission sequence of the new base station. Synchronisation, in turn, requires that the nearby base stations be monitored and listened to. In the GSM system, a mobile station regularly monitors several base stations and typically transmits to the network the signal strength information of the six best base stations. However, mobile stations do not employ two receivers, which would facilitate continuous monitoring of nearby base stations, because two receivers would mean considerably higher complexity and manufacturing costs for the mobile station. Therefore, the mobile station needs free time for carrying out measurements. In the GSM system this is solved by having a pause at every 26th frame on the TCH/F channel transferring speech so that the mobile station has about 6 ms to make measurements. Such a frame is called an idle frame. The pause is repeated at 120 ms intervals. In addition, during the same 120-ms period there are several pauses of about 1 ms and 2 ms between the transmission and reception turns of the mobile station and base station. In the GSM system, base stations transmit synchronisation data on special synchronisation (SCH) and frequency control (FCCH) channels. These channels are repeated at certain intervals in time slots 0. However, a mobile station connected with another base station may have a reception turn at the same time since in the GSM system the base stations are not synchronised with each other. Therefore, the mobile station needs a longer pause, or a whole idle frame to receive the synchronisation data, for which these short, 1-ms and 2-ms pauses are not long enough. These short pauses, however, can be used for measuring the signal strengths of the neighbouring base stations. The GSM system and its channel architecture, among other things, are discussed in greater detail e.g. in "The GSM System for Mobile Communications" by Michel Mouly and Marie-Bernadette Pautet, ISBN 2-9507190-0-7, Palaiseau 1992. However, the use of the idle frame does not guarantee a fast enough synchronisation to a new cell in new evolving mobile communication systems that will be discussed below.
New mobile communication systems under development such as the universal mobile telephone system (UMTS) put more demands on the mobile stations. For example, future systems will employ cells of several different hierarchy levels from very small microcells covering possibly only one block to macro cells covering tens of kilometers. In such a system, the smaller cells of lower hierarchy levels will be used by slowly-moving mobile stations and terminals requiring high transmission rates. Small cell size facilitates high transmission capacity and hence new services, such as transmission of video images. Microcells are densely located and typically overlapping, so typically a mobile station changing channels can choose from several cells that in practice offer connections of equally good quality. Mobile stations moving fast, e.g. in a car, use larger cells of a higher hierarchy level because if they used microcells they would be compelled to change base stations very frequently. In addition, macro cells cover the shadow areas possibly left by the microcells. Such a multiple-level cell system requires that a mobile station has to regularly monitor very many base stations on almost all cell hierarchy levels.
From the user's standpoint it is advantageous if the mobile station is capable of operating flexibly in more than one parallel mobile communication system, e.g. in the GSM system and in the DCS 1800 system. Presently, DCS 1800 systems are being constructed to alleviate the traffic problem in high-density areas, such as city centres. There may also be more than two parallel systems in the future. For a mobile station to be able to change from a base station of one system to a base station of another system it has to regularly monitor the nearby base stations of that other system as well. The monitoring is made more difficult by the fact that the systems may be synchronised in different ways: precisely synchronised, loosely synchronised within certain limits, or fully asynchronous. If the base station to be monitored belongs to an asynchronous system, the mobile station needs more measuring time than usual because it has no information on when the asynchronous base station will transmit the identification signal to be monitored.
In mobile communication systems, communications can be roughly categorised into two classes: real time (RT) connections and non-real time (NRT) connections. RT traffic typically consists of speech or e.g. video images where no interruptions or delays are allowed. NRT, or packet, communications may include e.g. file transfers where interruptions and delays are permitted. The RT class is typically divided into two sub-classes, namely, short-delay RT connections and long-delay RT connections. Typically, up to 100-ms delays are allowed in the long-delay RT connections. The long-delay RT connections are suitable for the transmission of video images, for example. The short-delay RT connections are used for transmitting speech. So, the quality of active connections affects the requirements on the mobile station in connection with a handover. If a mobile station has only NRT connections, a momentary interruption in conjunction with a handover is not harmful. If, however, there exist between the mobile station and the old base station one or more RT connections, the handover must be performed very quickly and without disturbances.
Solutions according to the prior art, such as those used in the GSM system, do not additionally facilitate monitoring of the necessary number of base stations as the network architecture of the mobile communication system gets more complex and as the parallel use of more than one mobile communication system becomes more general.